Currently, an active matrix organic light emitting diode (AMOLED) display is the focus of the display field due to its amazing image quality and optical specification better than the conventional display.
The AMOLED display is used as an emitter with current through an organic light emitting diode, the current is controlled by an active matrix, and brightness of gray scale is determined by an amount of the current during light emission.
The active matrix is composed of a group of pixel units, and an effective area of light emission is defined by resolution. The effective area of light emission is an area of pixel units multiplied by a resolution in a vertical direction and then multiplied by a resolution in a horizontal direction.
A typical pixel unit is composed of three sub-pixel units. Generally, a sub-pixel unit is composed of a plurality of thin film transistors and capacitors, a gray scale of emission brightness of a sub-pixel area is controlled by the thin film transistor, and the capacitor is used as a storage potential to stabilize driving current.
However, in comparison with other displays (for example, liquid crystal displays), since the active matrix organic light emitting diode display has a characteristic of current driving light emission, the brightness difference of the gray scale can be directly affected by component electrical properties of the thin film transistor. When the thin film transistors in different sub-pixels have too much difference in component electrical properties, an uneven image property would be formed. For example, mura phenomenon occurs.
Therefore, to overcome the above problem, a pixel compensation circuit is formed to compensate parameters (threshold voltage Vth, for example) of electrical properties of critical components, so as to repair the deterioration of the image quality due to the difference between characteristics of the components.
In addition, another major problem occurring in the current driving system is the voltage drop (IR-drop) effect, which is generated when the distal voltage drop is caused by the electrical load of the system. The large output current corresponds to large electrical load, such that for an active matrix organic light emitting diode (AMOLED) display which is typically designed as a common power source, the brightness near the power source end is higher than the brightness away from the power source end. The issue of brightness uniformity can be overcome by a compensation circuit.
However, as the progressiveness of display technologies, there are more and more pixels in one unit size, a component size for displaying each pixel is correspondingly reduced. At least three signals are required in the conventional pixel compensation circuit, such that at least three signal generators or wirings are required, so as to limit the size reduction.
Accordingly, the conventional technologies have many drawbacks and need to be improved. Therefore, the present invention provides a pixel compensation circuit to improve brightness of an AMOLED and reduce the number of the required control signals.